Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. Tang et al. in Applied Physics Letters 51, p913, 1987; Journal of Applied Physics, 65, p3610, 1989; and commonly assigned U.S. Pat. No. 4,769,292 demonstrated highly efficient OLEDs. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved. For the commercial success of OLEDs, further improvement in devices is needed. One of the key areas that needs further improvement is the operating stability of the devices.
The OLED devices today show a continuous degradation of luminance output with use. This gradual degradation of luminance output is unacceptable for many applications. Many approaches have been attempted to solve this degradation problem. One of the more promising approaches is to dope one of the organic layers with another material, generally referred to as a stabilizer, which stabilizes the devices. U.S. Patent Application Publication 2003/0068524 A1 disclosed improved OLED device stability by incorporating a rubrene dopant in the hole transport layer (HTL) next to a blue light-emitting layer (LEL). The detailed mechanism for this stabilizing effect is not well understood, but one important consequence of this approach is that the rubrene doped HTL also emits light, but with orange color, which combines with the blue light emitted from LEL to result in a white light emission from the OLED device. When a stabilizing dopant is added to an OLED device, the OLED device is stabilized, since the emitted light from the stabilizers most likely has a quite different spectrum from that of the OLED device. As a result, the emitted light from the OLED device is contaminated in color by the emission from the stabilizer. This contamination may not be acceptable for many practical applications.
One of the many approaches to achieve a full color OLED display is to use a color conversion approach. Color conversion OLED has been described by Tokailin et al. in commonly assigned U.S. Pat. No. 5,126,214. The color conversion OLED device is provided with a color conversion layer that includes a fluorescent material responsive to the color of light emitted by the light-emitting layer and is capable of changing the wavelength of the light and thereby re-emitting different colored light. It can be particularly useful to form an OLED device that generates light of a single hue (e.g., blue light) and include color conversion layers that convert the generated light into light of one or more different hues (e.g., green, red) that will be perceived by a viewer. Thus, it is possible to construct a full color OLED device with a light-emitting layer that produces only a single hue of light. Most commonly, the OLED display is designed to emit blue light. The subpixels designed to emit light of other hues are provided with florescent materials that can absorb the blue light emitted from the OLED and re-emit the other desired colors. With this approach, full color displays have many advantages, however, one of the key problems is that blue emitting OLEDs are generally the least stable and least efficient of OLED devices.
There is therefore a need to provide stable OLED devices and, in particular, blue light-emitting OLED devices without color contamination.